The present invention relates generally to brushless DC motors, and in particular to a control circuit for such motors. The invention is particularly suitable for driving motors used in audio or video systems wherein motor torque must be maintained to close tolerances.
Brushless DC motors and associated control circuits for use in audio and video systems are designed so as to minimize the amount of inherent mechanical vibrations to meet the imposed strict requirements. A typical example of such control circuits comprises at least one pair of Hall generators fixedly mounted with respect to the rotor. Each of the Hall generators is associated with a current control circuit which selectively supplies an input torque command signal to the armature windings in response to a voltage signal developed in the associated Hall generator to cause rotation of the rotor. The circuit includes a feedback path through which the amount of current flowing through the windings is subtractively combined with the input signal.
While the average value of torque is maintained constant due to the feedback operation, the instantaneous torque value varies as the rotor shifts from one armature winding to another. This ripple component is believed to arise from the fact that the feedback operation is only based upon the armature current which is not a true indication of the amount of torque needed to compensate for transient torque variations.